As protagonists of regional analgesia, we were interested to read
the article by Stevens et al (1). They have used morphine consumption
via patient controlled analgesia (PCA) as their primary outcome measure.
The anaesthetic literature is filled with similar studies (2,3). The
problem is that this does not show that the intervention improves
analgesia. Morphine PCA consumption is an investigator-driven outcome
variable and has thus been used repeatedly in acute pain studies. It is
easy to measure and relatively easy to show a statistical significance.
The principal outcome should surely rather relate to the patients'
experience of pain and secondary outcomes should include side-effects
from the pain or the analgesia. Unfortunately the variability in PCA
requirements is so large that this is not a reliable design
substitute(4).

The intervention (modified fascia lata block in this particular
study) does not show a significant improvement in the patient pain
experience. Nor does the study show a decrease in morphine-related side-
effects. The use of clonidine in the trial group clouds the matter
somewhat further, however that is not the point of this letter.

If an intervention reduces pain, one needs to have measurable
evidence that this is indeed the case. This is often difficult(4). It
can be done by independently measuring pain scores numerically or with
visual analogue scales (5,6). Farrar et al define "meaningful
improvement" as being a 30% reduction in numerical rating of pain
intensity. If morphine consumption is used as an objective measure, one
should compare patients in the two groups that have similar morphine
consumption and then look at their pain scores.

It is also useful to show reduced side-effects in the group that
has the intervention, for example, a decrease in morphine-associated
nausea and vomiting. However decreasing morphine consumption from, say,
35 mg to 25 mg over 24 hours in a particular patient is very unlikely to
decrease the incidence of nausea. One needs larger numbers of patients
or a dramatic decrease in opiate consumption to show decreases in nausea
and vomiting (6).

We believe it is important to consider this issue in future acute
pain intervention studies, whether the intervention is a regional
technique or the use of a new or different drug.

Birch and McLintic mention the difficulty of measuring pain and
question the validity of morphine consumption as a measure of the
utility of a pain intervention.

First, we designed our study on working lists in a regional
hospital with no registrar/research assistant. We needed a tool which
was objective, easy to measure and free from bias/inadvertant
manipulation. We believe that absolute morphine consumption is that
tool.

Second, we agree that numerical and visual analogue scales are
useful tools for clinically assessing pain in real time. These measures
are problematic as a method of assessing research data due to their
temporal variability. That is, if you ask the patient about their pain
just prior to PCA activation it is invariably higher than if you ask
them just after PCA activation. How often is this bias controlled for in
studies in the literature?

Our objective was to bring to the literature a modification of an
old block so that others may utilise it if they thought it would be of
benefit to the patient. It was both our clinical impression
(anaesthetists, nurses and patients) and the study's findings that
it would benefit patients undergoing total hip replacement by reducing
their morphine consumption.

M. STEVENS

G. HARRISON

M. MCGRAIL

Traralgon, Victoria

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